1. Field of the Invention
The present invention relates to solder alloys to be used in a solder joint between metallic parts of electronic equipment, especially to lead (Pb)-free solder alloys for avoiding the environmental or health effects of Pb.
2. Description of the Related Art
Heretofore, solders have been used to join metallic parts, such as electrical wiring in an assembly line of electrical equipment or the like. For the soldering process, a solder alloy should have excellent characteristics of wettability, ductility, thermal fatigue strength, corrosion resistance, and so on. In addition, consideration must be given to environmental pollution, so that the solder alloy should be prepared without using Pb. Regarding the health effects of Pb, for example, Pb in any form shows internal accumulative toxicity in mammal. Therefore, public concerns regarding Pb include air pollution, waste treatment in the lead smelting process, and accumulation in the interior of body of babies and pregnant women due to exposure to the air and contamination of foods and the like.
The conventional solder alloys include tin (Sn)-lead (Pb), tin (Sn)-silver (Ag), tin (Sn)-antimony (Sb), and tin (Sn)-bismuth (Bi) alloys. Among them, for example, a typical alloy known as 63Sn-37Pb (having a eutectic temperature of 183.degree. C.) contains 37% by weight of Pb based on the total amount of its composition and thus its use is not preferred because the of environmental effects of Pb.
Accordingly and for the following additional reasons, there is a need for providing a Pb-free solder alloy with a fluxing temperature of about 180.degree. C. First, the fluxing temperature is important because two or more solder alloys with different soldering temperatures may be used in two or more soldering steps for manufacturing intricate equipment. Second, there is also the need for insuring the reliability of semiconductor parts in consideration of thermal cycle resistance at a peak temperature of about 125.degree. C.
However each of the Pb-free alloys available to substitute for the Sn--Pb alloy has a comparatively high fluxing temperature. For example, the Sn--Sb alloy has a fluxing temperature of 232 to 245.degree. C. In addition, the Sn--Ag alloy has a eutectic temperature of 221 C. A solder alloy of Sn7.5Bi2Ag0.5Cu, which is one of the Sn--Bi solders, has a fluxing temperature of 200 to 220.degree. C. and requires a soldering temperature of 240 to 250.degree. C. Like the solder alloy of Sn7.5Bi2Ag0.5Cu, one that contains several percent of Bi is characterized by its low ductility in addition to the following problems. For example, the first problem is its poor processability and strength. The second problem is that a solid/liquid-coexisting region where a liquidus line and a solidus line exist together is broadened. Therefore, a delamination (i.e., a lift-off phenomenon) of soldered parts may occur as a result of uneven distribution of Bi when two parts are bonded together.
A tin (Sn)-indium (In) alloy prepared by a Sn-based alloy composition with an addition of In has been studied as a Pb-free solder alloy having a low fluxing temperature. The Sn--In alloy has a eutectic point of 118.degree. C., while a bismuth (Bi)-indium (In) alloy provided as another Pb-free solder alloy having a lower fluxing temperature has a eutectic point of 75.degree. C. However, heat-resisting temperatures of these Pb-free alloys are too low to be used in practice.